Сreating a child model causes a RecursionError exception

[pydevd]

Bug

• OS: Ubuntu 14.04
• Python version import sys; print(sys.version): 3.6.7
• Pydantic version import pydantic; print(pydantic.VERSION): 0.14

I'm expecting, that I can use a classic inheritance for dataclass models:

import pydantic.dataclasses

@pydantic.dataclasses.dataclass
class A:
    a: str = None

@pydantic.dataclasses.dataclass
class B(A):
    b: str = None

B(a='a', b='b')

But as a result I'm receiving this:

Traceback (most recent call last):
  File "/usr/local/pyenv/versions/3.6.7/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 2881, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-9-10a7116ca691>", line 12, in <module>
    B(a='a', b='b')
  File "<string>", line 4, in __init__
  File "/usr/local/pyenv/versions/3.6.7/lib/python3.6/site-packages/pydantic/dataclasses.py", line 13, in post_init
    self.__post_init_original__()
  File "/usr/local/pyenv/versions/3.6.7/lib/python3.6/site-packages/pydantic/dataclasses.py", line 13, in post_init
    self.__post_init_original__()
  File "/usr/local/pyenv/versions/3.6.7/lib/python3.6/site-packages/pydantic/dataclasses.py", line 13, in post_init
    self.__post_init_original__()
  [Previous line repeated 952 more times]
  File "/usr/local/pyenv/versions/3.6.7/lib/python3.6/site-packages/pydantic/dataclasses.py", line 9, in post_init
    d = validate_model(self.__pydantic_model__, self.__dict__)
  File "/usr/local/pyenv/versions/3.6.7/lib/python3.6/site-packages/pydantic/main.py", line 484, in validate_model
    v_, errors_ = field.validate(value, values, loc=field.alias, cls=model.__class__)
  File "/usr/local/pyenv/versions/3.6.7/lib/python3.6/site-packages/pydantic/fields.py", line 303, in validate
    v, errors = self._validate_singleton(v, values, loc, cls)
  File "/usr/local/pyenv/versions/3.6.7/lib/python3.6/site-packages/pydantic/fields.py", line 406, in _validate_singleton
    return self._apply_validators(v, values, loc, cls, self.validators)
  File "/usr/local/pyenv/versions/3.6.7/lib/python3.6/site-packages/pydantic/fields.py", line 412, in _apply_validators
    v = validator(v)
  File "/usr/local/pyenv/versions/3.6.7/lib/python3.6/site-packages/pydantic/validators.py", line 23, in str_validator
    if isinstance(v, (str, NoneType)):
RecursionError: maximum recursion depth exceeded in __instancecheck__

This line below causes this problem:
https://github.com/samuelcolvin/pydantic/blob/master/pydantic/dataclasses.py#L13

UPD: defining __post_init__ in child model fixes the problem, but this is workaround.

import pydantic.dataclasses

@pydantic.dataclasses.dataclass
class A:
    a: str = None

@pydantic.dataclasses.dataclass
class B(A):
    b: str = None

    def __post_init__():
        pass

B(a='a', b='b')

[samuelcolvin]

thanks for reporting, I'll try and fix soon.